The eight bits of a byte are sent one at a time

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Unformatted text preview: ufacturers other than the computer manufacturer can be easily plugged-in to the computer system. This provides more flexibility to the users in buying I/O devices of their choice. Device controllers may or may not have DMA (Direct Memory Access) capability. As shown in Figure 9.37, a device controller without DMA capability transfers data between the computer's memory and an I/O device via the CPU. On the other hand, a device controller with DMA capability can transfer data directly between the memory and an I/O device without the help of CPU. Obviously, device controllers with DMA capability can transfer data much faster than those without DMA capability. Due to this reason, nearly all device controllers of today have DMA capability. Serial and Parallel Ports We saw that I/O devices are connected to I/O ports of a device controller, which in turn is connected to the computer's system bus. The I/O ports are used to plug-in I/O devices to a computer system in a similar manner as electric sockets in a house are used to plug-in various types of electrical appliances when they need to be used. These I/O ports are basically of two types - serial ports and parallel ports. They are briefly described below. Serial Ports A serial port may be used to connect any device that can send or receive a stream of bytes serially, one bit at a time. A serial port is so called because it sends the data bits one at a time (that is, serially). It has only one wire for sending data and another one for receiving data. The eight bits of a byte are sent one at a time. Usually, two extra bits are sent with the byte to help in the control and timing of the communication. A chip called UART on the device controller's card converts parallel data from the system bus into serial data that can flow through a serial cable or telephone wire. The process is illustrated in Figure 9.38. Similarly, the serial data received by the device controller from the I/O device is converted into parallel data by the UART before it is sent to the system bus. Serial ports are used for connecting slow I/O devices such as terminal, keyboard, mouse, modem, etc. The physical appea...
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